Magnetic stack design

ABSTRACT

A magnetic stack having a free layer having a switchable magnetization orientation, a reference layer having a pinned magnetization orientation, and a barrier layer therebetween. The stack includes an annular antiferromagnetic pinning layer electrically isolated from the free layer and in physical contact with the reference layer. In some embodiments, the reference layer is larger than the free layer.

RELATED APPLICATION

This application claims priority to U.S. provisional patent applicationNo. 61/108,787, filed on Oct. 27, 2008 and titled “Memory Cell Structurefor STRAM”. The entire disclosure of application No. 61/108,787 isincorporated herein by reference.

BACKGROUND

Fast growth of the pervasive computing and handheld/communicationindustry has generated exploding demand for high capacity nonvolatilesolid-state data storage devices. Current technology like flash memoryhas several drawbacks such as slow access speed, limited endurance, andthe integration difficulty. Flash memory (NAND or NOR) also facesscaling problems. Also, traditional rotating storage (e.g., disc drives)faces challenges in areal density and in making components likereading/recording heads smaller and more reliable.

Resistive sense memories (RSM) are promising candidates for futurenonvolatile and universal memory by storing data bits as either a highor low resistance state. One such memory, MRAM, features non-volatility,fast writing/reading speed, almost unlimited programming endurance andzero standby power. The basic component of MRAM is a magnetic tunnelingjunction (MTJ). MRAM switches the MTJ resistance by using a currentinduced magnetic field to switch the magnetization of MTJ. As the MTJsize shrinks, the switching magnetic field amplitude increases and theswitching variation becomes more severe.

However, many yield-limiting factors must be overcome before resistivesense memory enters the production stage. One challenge is the magnitudeof the switching current in a resistive sense memory array. Inspin-torque transfer RAM (STRAM), this is dependent on several factorsincluding characteristics of the barrier layer. Therefore, a need existsfor designs that facilitate lower switching current.

BRIEF SUMMARY

The present disclosure relates to magnetic stacks (e.g., memory cellssuch as magnetic tunnel junction cells, and read sensors). Thestructures have a pinned reference layer configured for reducedinterlayer coupling between the reference layer and the free layer. Withthese structures, high tunneling magnetoresistance (TMR) can beachieved.

In one particular embodiment, this disclosure describes a magnetic stackhaving a free layer having a switchable magnetization orientation, areference layer having a pinned magnetization orientation, and a barrierlayer therebetween, each of the free layer, reference layer and barrierlayer having a center. The stack includes an annular antiferromagneticpinning layer having a center, with the center of the pinning layeraligned with the center of each of the free layer, reference layer andbarrier layer, the pinning layer electrically isolated from the freelayer and in physical contact with the reference layer.

In another particular embodiment, this disclosure describes a magneticstack having a free layer having a switchable magnetization orientation,a reference layer having a pinned magnetization orientation, and abarrier layer therebetween. The stack includes an antiferromagneticpinning layer electrically isolated from the free layer and in physicalcontact with the reference layer. Each of the free layer, referencelayer, barrier layer and pinning layer have a center and an outerdiameter, with the reference layer having a larger outer diameter thanthe free layer.

In yet another particular embodiment, this disclosure describes amagnetic stack having a free layer having a switchable magnetizationorientation, a synthetic antiferromagnetic (SAF) coupled reference layerhaving a pinned magnetization orientation, and a barrier layertherebetween. The SAF reference layer has a first ferromagnetic sublayerand a second ferromagnetic sublayer separated by a metallic spacer, withthe first sublayer different than the second sublayer.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1A is a cross-sectional schematic diagram of an illustrativemagnetic stack with in-plane magnetization orientation; FIG. 1B is across-sectional schematic diagram of an illustrative perpendicularanisotropy magnetic stack with out-of-plane magnetization orientation;

FIG. 2 is a schematic diagram of an illustrative memory unit including amemory cell and a semiconductor transistor;

FIG. 3 is a cross-sectional schematic diagram of an embodiment of amagnetic cell;

FIGS. 4A-4J illustrate a stepwise method for forming the magnetic cellof FIG. 3;

FIG. 5 is a cross-sectional schematic diagram of an embodiment of amagnetic cell;

FIG. 6 is a cross-sectional schematic diagram of an embodiment of amagnetic cell;

FIG. 7 is a cross-sectional schematic diagram of an embodiment of amagnetic cell;

FIGS. 8A-8H illustrate a stepwise method for forming the magnetic cellof FIG. 7; and

FIG. 9 is a cross-sectional schematic diagram of an embodiment of amagnetic cell.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

This disclosure is directed to magnetic stacks (e.g., spin torque memory(STRAM) cells, RRAM cells, and other resistive sense memory cells (RSMcells) and read sensors). The structures have a pinned ferromagneticreference layer, either a single layer or an SAF trilayer, that islarger than and extends past the ferromagnetic free layer. With such astructure, the interlayer coupling between the pinned reference layerand the free layer can be reduced, compared to a cell structure that hasthe same size for the reference layer and the free layer. Additionally,any electrical shorting issue at the edges of the ferromagnetic layersis inhibited. With these structures, high tunneling magnetoresistance(TMR) can be achieved. High TMR improves readability and writeability ofmemory arrays incorporated these memory cells.

In some embodiments, the magnetic cells include an annularantiferromagnetic pinning layer that is isolated from the free layer butin physical contact with the reference layer. In other embodiments, themagnetic cells include an asymmetric SAF trilayer.

In the following description, reference is made to the accompanying setof drawings that forms a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.Any definitions and descriptions provided herein are to facilitateunderstanding of certain terms used frequently herein and are not meantto limit the scope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the specification and attached claims areapproximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

It is noted that terms such as “top”, “bottom”, “above, “below”, etc.may be used in this disclosure. These terms should not be construed aslimiting the position or orientation of a structure, but should be usedas providing spatial relationship between the structures.

While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure and of the invention will be gainedthrough a discussion of the Figures and the examples provided below.

FIG. 1A is a cross-sectional schematic diagram of a magnetic cell 10Athat includes a soft ferromagnetic free layer 12A and a ferromagneticreference (i.e., pinned) layer 14A. Ferromagnetic free layer 12A andferromagnetic reference layer 14A are separated by an oxide barrierlayer 13A or non-magnetic tunnel barrier. Note that other layers, suchas seed or capping layers, are not depicted for clarity but could beincluded as technical need arises.

Reference layer 14A is larger in size than free layer 12A and extendspast the edges of ferromagnetic free layer 12A in at least onedirection, often in at least two opposite directions. For a circular ornearly circular cell, each layer (e.g., free layer 12A, reference layer14A, etc.) has a center point and an outer diameter. In someembodiments, reference layer 14A has a diameter greater than thediameter of free layer 12A, so that reference layer 14A extends pastfree layer 12A in all directions.

Ferromagnetic layers 12A, 14A may be made of any useful ferromagnetic(FM) material such as, for example, Fe, Co or Ni and alloys thereof,such as NiFe and CoFe. Ternary alloys, such as CoFeB, may beparticularly useful because of their lower moment and high polarizationratio, which are desirable for the spin-current switch. Either or bothof free layer 12A and reference layer 14A may be either a singleferromagnetic layer or a synthetic antiferromagnetic (SAF) coupledstructure, i.e., two ferromagnetic sublayers separated by a metallicspacer, such as Ru or Cu, with the magnetization orientations of thesublayers in opposite directions to provide a net magnetization. Themagnetization orientation of ferromagnetic free layer 12A is morereadily switchable than the magnetization orientation of ferromagneticreference layer 14A. Barrier layer 13A may be made of an electricallyinsulating material such as, for example an oxide material (e.g., Al₂O₃,TiO_(x) or MgO). Other suitable materials may also be used. Barrierlayer 13A could optionally be patterned with free layer 12A or withreference layer 14A, depending on process feasibility and devicereliability.

A first or bottom electrode 18A is in electrical contact withferromagnetic reference layer 14A and a second or top electrode 19A isin electrical contact with ferromagnetic free layer 12A. Electrodes 18A,19A electrically connect ferromagnetic layers 12A, 14A to a controlcircuit providing read and write currents through layers 12A, 14A.

Radially encircling at least free layer 12A, is an isolation layer 16A,which is electrically insulating. In this embodiment, isolation layer16A encircles free layer 12A, barrier layer 13A, and top electrode 19A.Isolation layer 16A has a thickness of about 2-30 nm and is formed ofelectrically insulating materials such as oxide(s) and nitride(s).Examples of suitable materials for isolation layer 16A include Si₃N₄,SiO₂, SiO_(x)N_(y), SiOCN, Ta₂O₅, Al₂O₃, MgO, and other low Kdielectrics. In other embodiments, isolation layer 16A encircles freelayer 12A and top electrode 19A.

The resistance across magnetic cell 10A is determined by the relativeorientation of the magnetization vectors or magnetization orientationsof ferromagnetic layers 12A, 14A. The magnetization direction offerromagnetic reference layer 14A is pinned in a predetermined directionwhile the magnetization direction of ferromagnetic free layer 12A isfree to rotate under the influence of spin torque. Pinning offerromagnetic reference layer 14A may be achieved through, e.g., the useof exchange bias with an antiferromagnetically ordered material such asPtMn, IrMn, and others.

Magnetic memory cell 10A is in the low resistance state when themagnetization orientation of free layer 12A is in the same direction(parallel) as the magnetization orientation of reference layer 14A.Conversely, a magnetic memory cell is in the high resistance state whenthe magnetization orientation of free layer 12A is in the oppositedirection (anti-parallel) of the magnetization orientation of referencelayer 14A. Switching the resistance state and hence the data state ofmagnetic cell 10A via spin-transfer occurs when a current, passingthrough a magnetic layer of magnetic cell 10A, becomes spin polarizedand imparts a spin torque on free layer 12A. When a sufficient spintorque is applied to free layer 12A, the magnetization orientation offree layer 12A can be switched between two opposite directions andaccordingly, magnetic cell 10A can be switched between the lowresistance state and the high resistance state.

The magnetization orientations of free layer 12A and reference layer 14Aof magnetic memory cell 10A are in the plane of the layers, or“in-plane”. FIG. 1B illustrates an alternate embodiment of a magneticmemory cell that has the magnetization orientations of the free layerand the pinned layer perpendicular to the plane of the layers, or“out-of-plane”.

Similar to magnetic cell 10A of FIG. 1A, magnetic cell 10B of FIG. 1Bhas soft ferromagnetic free layer 12B and a ferromagnetic reference(i.e., pinned) layer 14B separated by an oxide bsarrier layer 13B ornon-magnetic tunnel barrier. A first or bottom electrode 18B is inelectrical contact with ferromagnetic reference layer 14B and a secondor top electrode 19B is in electrical contact with ferromagnetic freelayer 12B. Electrodes 18B, 19B electrically connect ferromagnetic layers12B, 14B to a control circuit providing read and write currents throughlayers 12B, 14B. An electrically insulating radial isolation layer 16Bencircles at least free layer 12A and top electrode 19B. The variouselements of cell 10B are similar to the elements of cell 10A, describedabove, except that the magnetization orientations of layers 12B, 14B areoriented perpendicular to the layer extension rather than in the layerplane.

Free layer 12B and reference layer 14B each have a magnetizationorientation associated therewith, illustrated in FIG. 1B. In someembodiments, magnetic cell 10B is in the low resistance state where themagnetization orientation of free layer 12B is in the same direction(parallel) as the magnetization orientation of reference layer 14B. Inother embodiments, magnetic cell 10B is in the high resistance statewhere the magnetization orientation of free layer 12B is in the oppositedirection (anti-parallel) as the magnetization orientation of referencelayer 14B.

Similar to cell 10A of FIG. 1A, switching the resistance state and hencethe data state of magnetic cell 10B via spin-transfer occurs when acurrent, passing through a magnetic layer of magnetic cell 10B, becomesspin polarized and imparts a spin torque on free layer 12B. When asufficient spin torque is applied to free layer 12B, the magnetizationorientation of free layer 12B can be switched between two oppositedirections and accordingly, magnetic cell 10B can be switched betweenthe low resistance state and the high resistance state.

Both memory cells 10A, 10B are illustrated with undefined magnetizationorientations for free layer 12A, 12B. As indicated above, a magneticmemory cell is in the low resistance state when the magnetizationorientation of free layer 12A, 12B is in the same direction as themagnetization orientation of reference layer 14A, 14B. Conversely, amagnetic memory cell is in the high resistance state when themagnetization orientation of free layer 12A, 12B is in the oppositedirection of the magnetization orientation of reference layer 14A, 14B.In some embodiments, the low resistance state is the “0” data state andthe high resistance state is the “1” data state, whereas in otherembodiments, the low resistance state is “1” and the high resistancestate is “0”.

The magnet stack of memory cells 10A, 10B of FIGS. 1A and 1B can also beused as a magnetic read sensor in a hard disc drive with somemodifications. In such uses, free layer 12A, 12B is influenced by astored magnetic state on an adjacent recording media, and when a currentis passed through the stack, the magnetization orientation in the mediacan be detected.

FIG. 2 is a schematic diagram of an illustrative memory unit 20including a memory element 21 electrically coupled to a semiconductortransistor 22 via an electrically conducting element. Memory element 21may be any of the memory cells described herein. Transistor 22 includesa semiconductor substrate 25 having doped regions (e.g., illustrated asn-doped regions) and a channel region (e.g., illustrated as a p-dopedchannel region) between the doped regions. Transistor 22 includes a gate26 that is electrically coupled to a word line WL to allow selection andcurrent to flow from a source line SL to memory element 21 and bit lineBL. An array of programmable metallization memory units 20 can be formedon a semiconductor substrate with word lines and bit lines utilizingsemiconductor fabrication techniques. Both memory cell 10A of FIG. 1Aand memory cell 10B of FIG. 1B are illustrated connected to a bit lineBL via their top electrode 19A, 19B.

FIG. 3 illustrates a first embodiment of a memory cell having a pinnedSAF trilayer reference layer that is larger than and extends past theferromagnetic free layer. This embodiment includes an antiferromagneticpinning layer that is isolated from the free layer but in physicalcontact with the reference layer. The antiferromagnetic pinning layer isannular in some embodiments. In particular, memory cell 30 has a softferromagnetic free layer 32 and a SAF trilayer reference (i.e., pinned)layer 34 separated by a barrier layer 33. In some embodiments, barrierlayer 33 is an oxide barrier layer, in other embodiments it can be anon-magnetic tunnel barrier. In one embodiment, trilayer 34 is composedof two layers of ferromagnetic material (e.g., CoFeB) separated by alayer of Ru, and barrier layer 33 is composed of MgO. A first or bottomelectrode 38 is in electrical contact with trilayer 34 and a second ortop electrode 39 is in electrical contact with ferromagnetic free layer32. An electrically insulating isolation layer 36 encircles free layer32 and top electrode 39. The various elements of cell 30 are similar tothe element of cells 10A, 10B described above, except as noted.

Memory cell 30 also includes a hard mask 37 positioned above topelectrode 39. In some embodiments, hard mask 37 is electricallyconducting and is integral with or replaces top electrode 39. Memorycell 30 also includes an antiferromagnetic pinning layer 35 radiallyencircling the stack of barrier layer 33, free layer 32 and topelectrode 39 and electrically insulated therefrom by isolation layer 36.In the illustrated embodiment, a portion of isolation layer 36 isexposed and not encircled by pinning layer 35. Free layer 32 isphysically and electrically isolated from pinning layer 35, which is inphysical contact with trilayer 34 at its extended area (i.e., proximatethe outer diameter of trilayer 34) and provides pinning for SAF trilayer34.

For memory cell 30, SAF trilayer 34 is larger than and extends past freelayer 32; that is, trilayer 34 has a larger outer diameter than freelayer 32. Trilayer 34 also is larger than and extends past barrier layer33, which in turn is larger than and extends past free layer 32.Trilayer 34, barrier 33 and free layer 32 are stacked with their centersaligned.

The process flow to make this memory cell stack structure is shown inFIGS. 4A-4J. First, in FIG. 4A, a stack of appropriate materials formingbottom electrode 48, SAF trilayer 44, barrier layer 43, free layer 42,and top electrode 49 is deposited. At this stage, high-temperaturethermal annealing is done to induce the epitaxial formation in barrierlayer 43 (e.g., MgO barrier layer) and crystallization of ferromagneticfree layer 42 and SAF trilayer 44. In FIG. 4B, a hard mask 47 isdeposited on to top electrode 49 and then patterned. Subsequently, viamilling and etching, free layer 42 is patterned and the etching isstopped at barrier layer 43. A protective layer 46, e.g., siliconnitride, is deposited in FIG. 4C to cover the stack of FIG. 4B. Aftermilling and etching the structure of FIG. 4C, the extended area of SAFtrilayer 44 is exposed in FIG. 4D while barrier layer 43 remains coveredby protective isolation layer 46.

Then in FIG. 4E, an antiferromagnetic pinning layer 45 is deposited overthe structure of FIG. 4D in contact with SAF trilayer 44 at the exposedarea. In some embodiments, a very thin ferromagnetic layer may bedeposited over the structure of FIG. 4D before deposition ofantiferromagnetic pinning layer 45 to increase the pinning effect. Nextin FIG. 4F, milling is performed to trim antiferromagnetic pinning layer45 to achieve physical, electrical and magnetic separation between hardmask 47 and antiferromagnetic pinning layer 45. In some embodiments, forexample, if pinning layer 45 is an insulator (e.g., NiO), thisseparation is not necessary. However, many pinning materials aremetallic and are alloys of Mn, thus the separation is desired.

In FIG. 4G the entire stack is annealing at elevated temperature in thepresence of a strong external in-plane magnetic field. If Mn is presentin pinning layer 45, this annealing should be within a temperature rangeso that Mn diffusion is controlled. The magnetic field will align themagnetizations of SAF trilayer 44 in the field direction and will alsoinduce exchange bias (pinning) of the top ferromagnetic layer oftrilayer 44 (that is in contact with antiferromagnetic pinning layer45). When the field annealing is completed, the resulting SAF trilayer44 is pinned by antiferromagnetic layer 45.

The memory cell is finalized by deposition of dielectric material 40 inFIG. 4H to encase the structure. This dielectric material 40 is polishedin FIG. 41 to provide a planar surface, and a bit line BL is depositedand patterned on top electrode 49 and hard mask 47 in FIG. 4J.

FIG. 5 illustrates another embodiment of a memory cell having a pinnedreference layer that is larger than and extends past the ferromagneticfree layer and having an annular pinning layer. In particular, memorycell 50 has a soft ferromagnetic free layer 52 and a single layerreference (i.e., pinned) layer 54 separated by an oxide barrier layer 53or non-magnetic tunnel barrier. A first or bottom electrode 58 is inelectrical contact with reference layer 54 and a second or top electrode59 is in electrical contact with ferromagnetic free layer 52. A hardmask 57 is positioned above top electrode 59. An electrically insulatingradial isolation layer 56 encircles free layer 52, hard mask 57 and topelectrode 59. An antiferromagnetic pinning layer 55 radially encirclesat least a portion of isolation layer 56 and the stack of barrier layer53, free layer 52, top electrode 59 and hard mask 57. The variouselements of cell 50 are similar to the element of cells 10A, 10B, 30described above, except as noted.

In this embodiment of memory cell 50, reference layer 54 is larger thanand extends past barrier layer 53, which in turn is larger than andextends past free layer 52. Reference layer 54, barrier 53 and freelayer 52 are stacked with their centers aligned.

FIG. 6 illustrates another embodiment of a memory cell having a pinnedreference layer that is larger than and extends past the ferromagneticfree layer and that has an asymmetric SAF trilayer instead of a pinninglayer. In particular, memory cell 60 has a soft ferromagnetic free layer62 and an SAF trilayer reference (i.e., pinned) layer 64 separated by anoxide barrier layer 63 or non-magnetic tunnel barrier. A first or bottomelectrode 68 is in electrical contact with trilayer 64 and a second ortop electrode 69 is in electrical contact with ferromagnetic free layer62. A hard mask 67 is positioned above top electrode 69. An electricallyinsulating radial isolation layer 66 encircles free layer 62, hard mask67 and top electrode 69. The various elements of cell 60 are similar tothe element of cells 10A, 10B, 30, 50 described above, except as noted.

Unlike memory cell 30 of FIG. 3 and cell 50 of FIG. 5, memory cell 60has no antiferromagnetic layer for pinning. Rather, trilayer 64(composed of a first ferromagnetic layer 64A, a metallic spacer 64B, anda second ferromagnetic layer 64C) is asymmetric in either physicalthickness or coercivity between its ferromagnetic layers. That is,ferromagnetic layers 64A and 64C either have a different physicalthickness or have a different coercivity. In FIG. 6, layer 64A isillustrated physically thicker than layer 64C. The magnetizationconfiguration and orientation of trilayer 64 are defined after magneticfield setting. With no antiferromagnetic pinning layer in cell 60,trilayer 64 is designed in shape to induce shape anisotropy againstthermal activation.

FIG. 7 illustrates another embodiment of a memory cell having a pinnedreference layer that is larger than and extends past the ferromagneticfree layer and having an annular pinning layer. In particular, memorycell 70 has a soft ferromagnetic free layer 72 and an SAF trilayerreference (i.e., pinned) layer 74 separated by an oxide barrier layer 73or non-magnetic tunnel barrier. A first or bottom electrode 78 is inelectrical contact with trilayer 74 and a second or top electrode 79 isin electrical contact with ferromagnetic free layer 72. A hard mask 77is positioned above top electrode 79. An electrically insulating radialisolation layer 76 encircles free layer 72, hard mask 77 and topelectrode 79. The various elements of cell 70 are similar to the elementof cells 10A, 10B, 30, 50, 60 described above, except as noted.

Memory cell 70 includes a pinning layer 75 positioned below the stack offree layer 72, barrier layer 73, and trilayer 74. In this embodiment,pinning layer 75 is an annular ring at the outer periphery of bottomelectrode 78, in physical contact with and exchange coupled withtrilayer 74. In this embodiment, pinning layer 75 is centered around thestack of free layer 72, barrier layer 73, and trilayer 74 and does notvertically overlap or intersect with the stack.

The process flow to make memory cell 70 is shown in FIGS. 8A-8H. First,in FIG. 8A, a metal layer 80 deposited, which will form the eventualbottom electrode. Metal layer 80 is masked and patterned (e.g., milled)to form bottom electrode 88 of FIG. 8B. In FIG. 8C, antiferromagneticmaterial is deposited in a ring around bottom electrode 88 and thenpolished to form pinning layer 85. An SAF trilayer 84, a barrier layer83, a free layer 82 and top electrode 89 are sequentially formed in FIG.8D over bottom electrode 88 and pinning layer 85. At this step, thevarious layers of pinning layer 85/bottom electrode 88, SAF trilayer 84,barrier layer 83, free layer 82 and top electrode 89 in the stack havethe same diameter. In FIG. 8E, free layer 82 and top electrode 89 aremasked with hard mask 87 and patterned, to have a reduced size inrelation to SAF trilayer 84 and barrier layer 83. Isolation material 86is deposited in FIG. 8F to cover and encase the structure of FIG. 8E.This isolation material 86 is optionally milled and then covered with adielectric material, which is polished in FIG. 8G to provide a planarsurface of hard mask 87 and isolation material 86. A bit line BL isdeposited and patterned on top electrode 89 and hard mask 87 in FIG. 8H.An alternate to memory cell 70 of FIG. 7, with a single pinned referencelayer, is shown in FIG. 9. Memory cell 90 of FIG. 9 has a softferromagnetic free layer 92 and a single layer reference (i.e., pinned)layer 94 separated by an oxide barrier layer 93 or non-magnetic tunnelbarrier. A first or bottom electrode 98 is in electrical contact withlayer 94 and a second or top electrode 99 is in electrical contact withfree layer 92. A hard mask 97 is positioned above top electrode 99. Anelectrically insulating radial isolation layer 96 encircles free layer92, hard mask 97 and top electrode 99. An annular pinning layer 95 ispositioned below the stack of free layer 92, barrier layer 93, and layer94. The various elements of cell 90 are similar to the element of cells10A, 10B, 30, 50, 60, 70 described above, except as noted.

The structures of this disclosure, including any or all of the magneticcells, may be made by thin film techniques such as chemical vapordeposition (CVD), physical vapor deposition (PVD), and atomic layerdeposition (ALD). Material removal may be by etching, including milling,ion beam milling, wet etching, and the like.

Thus, embodiments of the MAGNETIC STACK DESIGN are disclosed. Theimplementations described above and other implementations are within thescope of the following claims. One skilled in the art will appreciatethat the present disclosure can be practiced with embodiments other thanthose disclosed. The disclosed embodiments are presented for purposes ofillustration and not limitation, and the present invention is limitedonly by the claims that follow.

1. A magnetic stack comprising: a ferromagnetic free layer having aswitchable magnetization orientation, a ferromagnetic reference layerhaving a pinned magnetization orientation, and a barrier layertherebetween, each of the free layer, reference layer and barrier layerhaving a center; and an annular antiferromagnetic pinning layer having acenter, with the center of the pinning layer generally aligned with thecenter of each of the free layer, reference layer and barrier layer, thepinning layer electrically isolated from the free layer and in physicalcontact with the reference layer.
 2. The magnetic stack of claim 1wherein the reference layer has a larger area than the free layer. 3.The magnetic stack of claim 1 wherein the reference layer is a syntheticantiferromagnetic (SAF) trilayer.
 4. The magnetic stack of claim 1wherein the reference layer is a single ferromagnetic pinned layerlayer.
 5. The magnetic stack of claim 1 wherein the annular pinninglayer encircles at least the free layer.
 6. The magnetic stack of claim5 further comprising an electrically insulating isolation layer betweenthe annular pinning layer and the free layer.
 7. The magnetic stack ofclaim 1 wherein the annular pinning layer is positioned on a side of thepinned layer opposite the free layer.
 8. The magnetic stack of claim 1wherein the barrier layer has a larger area than the free layer.
 9. Themagnetic stack of claim 1 wherein the magnetic stack is a magnetictunnel junction memory cell.
 10. A magnetic stack comprising: aferromagnetic free layer having a switchable magnetization orientation,a ferromagnetic reference layer having a pinned magnetizationorientation, and a barrier layer therebetween, and an antiferromagneticpinning layer electrically isolated from the free layer and in physicalcontact with the reference layer; with each of the free layer, referencelayer, barrier layer and pinning layer having a center and an outerdiameter, with the reference layer having a larger outer diameter thanthe free layer.
 11. The magnetic stack of claim 10 wherein the barrierlayer has a larger outer diameter than the free layer.
 12. The magneticstack of claim 10 wherein the reference layer is a syntheticantiferromagnetic (SAF) trilayer.
 13. The magnetic stack of claim 10wherein the pinning layer is annular and has an inner diameter.
 14. Themagnetic stack of claim 13 wherein the inner diameter of the pinninglayer is larger than the outer diameter of the free layer.
 15. Themagnetic stack of claim 10 further comprising an electrically insulatingisolation layer between the annular pinning layer and the free layer.16. The magnetic stack of claim 10 wherein the annular pinning layer ispositioned on a side of the pinned layer opposite the free layer.
 17. Amagnetic stack comprising: a ferromagnetic free layer having aswitchable magnetization orientation, a synthetic antiferromagnetic(SAF) coupled reference layer having a pinned magnetization orientation,and a barrier layer therebetween, the SAF reference layer comprising afirst ferromagnetic sublayer and a second ferromagnetic sublayerseparated by a metallic spacer, the first sublayer different than thesecond sublayer.
 18. The magnetic stack of claim 17 wherein the firstsublayer is thicker than the second sublayer.
 19. The magnetic stack ofclaim 17 wherein the first sublayer has a different coercivity than thesecond sublayer.
 20. The magnetic stack of claim 17 further comprisingan electrically insulating isolation layer encircling the free layer.